class Solution {
    int m, n;
    int dx[4] = { 0, 0, -1, 1 };
    int dy[4] = { 1, -1, 0, 0 };
public:
    int numIslands(vector<vector<char>>& grid) {
        m = grid.size(), n = grid[0].size();

        int ret = 0;
        for (int i = 0; i < m; i++)
        {
            for (int j = 0; j < n; j++)
            {
                if (grid[i][j] == '1')
                {
                    ret++;
                    dfs(grid, i, j);
                }
            }
        }

        return ret;
    }

    void dfs(vector<vector<char>>& grid, int i, int j)
    {
        for (int k = 0; k < 4; k++)
        {
            int x = i + dx[k], y = j + dy[k];
            if (x >= 0 && x < m && y >= 0 && y < n && grid[x][y] == '1')
            {
                grid[x][y] = '2';
                dfs(grid, x, y);
            }
        }
    }
};